The field of the invention is nuclear magnetic resonance imaging ("MRI") methods and systems. More particularly, the invention relates to the acquisition of MRI data during a readout gradient that varies in strength.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated and this signal may be received and processed to forman image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. The concept of acquiring NMR image data in a short time period has been known since 1977 when the echo-planar pulse sequence was proposed by Peter Mansfield (J. Phys. C.10: L55-L58, 1977). In contrast to standard pulse sequences, the echo-planar pulse sequence produces a set of NMR signals for each RF excitation pulse. These NMR signals can be separately phase encoded so that an entire scan of 64 views can be acquired in a single pulse sequence of 20 to 100 milliseconds in duration. The advantages of echo-planar imaging ("EPI") are well-known, and there has been a long felt need for apparatus and methods which will enable EPI to be practiced in a clinical setting.
A characteristic of the EPI pulse sequence and many other fast pulse sequences is that the magnetic field gradient applied while-the NMR signal is acquired (i.e., the "readout" gradient) is switched on and off at a very high rate. Indeed, the inability to produce uniform readout gradient fields over very short time intervals has limited the clinical application of EPI and other fast pulse sequences. Due to gradient power supply limitations, gradient coil inductance and FDA limitations, a typical short readout gradient pulse will ramp up in value, plateau for a short interval, and then ramp down to zero. Since the resolution along the readout axis is determined by the area beneath of the readout gradient and the rate at which the NMR signal is sampled, the usual practice is to sample the NMR signal only after the readout gradient has ramped up to its specified constant value. The resulting delay in acquiring the NMR signal is very significant in fast pulse sequences. An alternative approach proposed by Avideh Zakhor, et al. "Optimal Sampling and Reconstruction of MRI Signals Resulting from Sinusoidal Gradients, " IEEE Trans. on Sig. Proc., Vol 39, No. 9, pp. 2056-65 (1991) is to vary the sample rate as a function of readout gradient strength, but this requires special receiver hardware and it does not provide optimal SNR or filtering to prevent aliasing of signals outside the desired field of view into the image.